High accuracy and high stability magnetic displacement sensor in the presence of electromagnetic interferences

Abstract

An accurate and stable displacement sensor that reads through coated metal substrates achieves better than one micron accuracy includes: an electromagnetic coil positioned in a first enclosure; (ii) means for generating a magnetic field from the electromagnetic coil; (iii) a second enclosure which is spaced apart from the first enclosure, wherein the second enclosure includes dual magnetic sensors, such as fluxgate sensors, that are configured to measure the magnetic field; and (iv) means for calculating the separation between the operative surfaces of the enclosures from magnetic field measurements. A permanent magnet can be used instead of the electromagnetic coil and associated driving energy source. A precise displacement measurement is given by a mathematical function (such as the ratio or difference) of the two magnetic sensors demodulated signals. The displacement sensor can be mounted on a maneuverable C-frame to monitor the caliper of anodes and cathodes produced for lithium ion batteries.

Description

FIELD OF THE INVENTION

[0001] The present invention generally relates to scanner measurement systems for determining parameters of continuous sheet materials and, more particularly, to non-contacting thickness or caliper measurement techniques of sheet materials containing metal such as coated metal foils and substrates used to manufacture anodes and cathodes for electrochemical cells and batteries.BACKGROUND OF THE INVENTION

[0002] Numerous methods exist for measuring the thickness of a moving web or sheet. For instance, a non-contact laser caliper apparatus comprises a laser source on either side of the web, whose light is directed onto the web surface and subsequently reflected to a receiver. The characteristics of the received laser signal are thereafter used to determine the distance from each receiver to the web surface. These distances are added together, and the result is subtracted from a known value for the distance between the two laser receivers. The result represents the web's thickness.

[0003] To compensate for the possibility of changes in the distance between the two laser sensors, prior art systems incorporate an eddy current sensor to detect the distance between the two laser sensors. Typically, the eddy current sensor comprises of an RF coil at an upper sensor enclosure that is fixed with respect with a top sensor and metal target at the lower sensor closure that is fixed with respect to the lower laser sensor. These non-contact devices are suitable for measuring paper and plastic but not for measuring conductive materials such as coated substrates used in fabricating anodes and cathodes for lithium ion batteries.

[0004] The art is in need of an accurate and repeatable technique for measuring the thickness and related properties of coatings and films that are formed on continuous, traveling non-uniform webs made of metal containing materials.

[0005] Examples of currently used systems can be found in the following: US2005 / 157314 discloses a device for measuring the thickness of a sheet or web product including at least one moveable measuring sensor which can be pressed against the product while forming an air pad between measuring sensor and product. At least one measuring sensor is respectively provided on both sides of the sheet or web product. The thickness of the respective air pad can be measured, the total gap thickness including the thickness of the two air pads and the product thickness can be measured. Elements are provided for calculating the product thickness by subtracting the thickness of the air pad on the two product sides from the measured total gap thickness.

[0006] US2016 / 123773 discloses a gap and displacement magnetic sensor system for scanner heads in paper machines or other systems includes a multiple-sensor assembly. The multiple-sensor assembly includes multiple magnetic field orientation sensors configured to capture measurements of a magnetic field in order to identify (i) a displacement of first and second scanning sensor heads in a first direction, and (ii) a gap separation of the first and second scanning sensor heads in a second direction, and (iii) a displacement of the first and second scanning sensor heads in a third direction. At least one of the magnetic field orientation sensors is disposed offset from a centerline of the magnetic field such that an output from the at least one magnetic field orientation sensor indicates a combination of the gap separation and the displacement in either the first direction or the third direction.

[0007] US6586930 discloses an apparatus for and a method of measuring material thickness with magnetics. The thickness monitoring system includes a thickness monitor, a probe, and a target. In a preferred embodiment, the probe is positioned on one side of an article for which the thickness is to be determined. The target is positioned on the opposite side of the article from the probe. The probe includes an excitation coil, a field compensation coil, and a magnetic sensor. The method includes energizing the excitation coil to excite a response from the target, compensating for the effect of the excitation coil on the magnetic sensor, measuring the response of the target with the magnetic sensor, and determining the thickness of the article from the measured response. The preferred mode of energizing the excitation coil is with an AC waveform; however DC, multi-frequency AC, or a combination of AC and DC waveforms may be used.

[0008] US2019 / 285527 discloses time-of-flight measurements calculate the absolute caliper of a moving film independent of the film's index of refraction. A reflective fiber coupled terahertz gauge is mounted co-axially with a temperature stabilized Z-sensor positioned within a scanner head. The terahertz gauge monitors four reflections: the reflection from a sensor window, the reflections from the top and bottom surfaces of the sheet product being measured, and the reflection from a reflector that is placed behind the sheet. The Z-sensor monitors the distance between the reflector and the senor window. The terahertz reflection delays together with the Z distance measurements allow extraction of the caliper. Since the time delay due to the sheet is a function of thickness and index of refraction, the basis weight of the sheet can be determined by using a calibration of the sensor relating basis weight of the product to time delay.

[0009] WO2020 / 068711 discloses a sensor system includes an eddy current sensor including at least one coil with excitation electronics coupled across the coil. An optical displacement sensor is secured to the eddy current sensor so that a vertical distance between the sensors is fixed. The optical displacement sensor is located on top of and concentric with the coil so that a measurement axis of the optical displacement sensor is collinear with an axis of symmetry of the coil. A computing device including a processor and memory is coupled to receive sensor data from the eddy current sensor and the optical displacement sensor that is adapted for analyzing the sensor data obtained from measuring a coated substrate including a coating layer on at least one side of a metal substrate to determine at least a thickness of the coating layer.

[0010] US 4 678 915 discloses a system for measuring the values of a parameter of a sheet of material is provided. The system includes a head system with sensors mounted therein and a distance correction system to correct the measured parameter for variations in the distance between parts of the head system.SUMMARY OF THE INVENTION

[0011] The present invention in its various aspects is set out in the appended claims. The present invention is based in part on the development of a high accuracy and high stability magnetic absolute displacement sensor that measures the distance between the top and bottom scanning head. The displacement sensor reads through metal materials such as coated metal substrates used in fabricating conductive anodes and cathodes suitable for lithium ion electrochemical cells and batteries. The displacement sensor exhibits better than I micron accuracy.

[0012] In one aspect, the invention is directed to a magnetic absolute displacement sensor that includes: a source of a magnetic field that is positioned in a first enclosure that defines a first operative surface; a second enclosure that defines a second operative surface which is spaced apart from the first operative surface, wherein the second enclosure includes a first magnetic sensor that is configured to measure the magnetic field and a second magnetic sensor that is configured to measure the magnetic field; and means for calculating the separation between the first and second operative surfaces from magnetic field measurements from the first and second magnetic sensors.

[0013] A permanent magnet can be used to generate the magnetic field. Alternatively, an electromagnetic coil that is driven by direct or alternating current can be used to generate the magnetic field in the first enclosure.

[0014] In another aspect, the invention is directed to a system for monitoring a property of a sheet of material that can contain metal wherein the sheet has a first side and a second side which includes: a first member disposed adjacent to the first side of the sheet of material, the first member having means for producing a magnetic field; a second member disposed adjacent to the second side of the sheet of material, the second member having a first magnetic sensor that detects the magnetic field and generates a first electrical signal and a second magnetic sensor that detects the magnetic field and generates a second electrical signal; and means for analyzing the first electrical signal and second electrical signal to determine changes in a distance between the first and second members.

[0015] In yet another aspect, the invention is directed to a method of measuring the thickness of a web having a first side and a second side that includes: providing a first distance sensor on the first side of the web; determining the position of the first distance sensor relative to the first side of the web with the first distance sensor; providing a second distance sensor on the second side of the web; determining the position of the second distance sensor relative to the second side of the web with the second distance sensor; generating a magnetic field at a fixed location relative to the first distance sensor on the first side of the web; positioning first and second magnetic sensors at respective fixed first and second locations to the second distance sensor on the second side of the web; measuring the magnetic field with the first and second magnetic sensors; and determining the thickness of the web from measurements of the magnetic field by the first and second magnetic sensors.

[0016] The magnetic displacement sensor is preferably incorporated into an online scanning system wherein the sheet being monitored travels between the dual scanner heads enclosing the electromagnetic coil and magnetic sensors. The dual scanner heads traverse back and forth along the cross direction relative to the sheet, which typically has a thickness of 0.005 to 5 mm. One embodiment of the scanning system employs a slidably moveable C-frame structure with dual arms or members to which the two scanner heads are attached. With the C-frame, the magnetic sensors and other sensors can be mounted directly onto the elongated members so that scanner heads are not needed. In this configuration, a permanent magnet can be integrated into or attached to one of the elongated members that is opposite the other member with the dual magnetic field sensors. The magnetic sensors can be calibrated by positioning a target sample of known thickness in the measurement gap or channel between the upper and lower scanner heads. The target sample can be a foil or standardization tile. A target sample is measured by the optical displacement sensors. The gap is OD1 + OD2 + t, where ODx are the optical displacement readings and t is the thickness. This can be compared to the reading from the magnetic sensors.

[0017] The gap size is then adjusted by placing a series of weights on the upper arm of the C-frame structure. The gap size adjustments are detected by the optical displacement sensors. A curve or mathematical function is constructed from the data using curve-fitting techniques. The curve or parametric equation is the calibration that correlates a mathematical operation of the readings from the two magnetic sensors to the size or distance of the gap.

[0018] The present invention is particularly suited for quality control in the production of anodes and cathodes for lithium ion cells and batteries. In making these electrodes, a metal substrate or foil is coated with an anode or cathode composition and the coated foil is then processed in a press section of an assembly process which controls the final caliper of the electrode. Caliper or thickness is a critical electrode specification.

[0019] The inventive displacement sensor reads through the electrodes which typically consist of copper or aluminum substrates. To meet lithium ion battery specifications, an accuracy of better than 1 micron is required. Conventional eddy current sensors cannot be used because of the conductive electrodes. In addition, displacement sensor, which employs two independent magnetic sensors that measure a magnetic field, exhibits the accuracy and repeatability required. Furthermore, it is not highly sensitive to interfering magnetic fields produced by power lines, motors and steel rolls.

[0020] It has been demonstrated that a 1KHz sinusoidal magnetic field of a few tens of mTesla can be produced by an electromagnetic coil positioned in an upper scanner head. The time varying magnetic field can be sensed by two magnetic sensors, typically two fluxgate sensors, that are positioned a few centimeters apart in the lower scanner head. The signal from the magnetic sensors is demodulated using the 1KHz coil signal as a sync signal. A precise displacement measurement is given by a mathematical function (such as the ratio or difference) of the two magnetic sensors demodulated signals.

[0021] The 1 KHz modulation / demodulation scheme produces a signal with high signal to noise while filtering the effect of interfering magnetic fields either static or time varying. The ratio (or difference) of the two magnetic sensor outputs cancels the effect of variation in current flowing through the coil and provides a highly stable and reliable measurement.BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Fig. 1 is a schematic of scanner heads of a thickness measurement apparatus; Fig. 2 shows the operation of the thickness measurement apparatus; Fig. 3 depicts an online scanning system with scanner heads secured to C-frame structure that is mounted on the translation mechanism; Fig. 4 depicts another online scanning system; and Fig. 5 is a graph of magnetic flux density vs. distance along a centerline as calculated from the Biot-Savart law. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Fig. 1 illustrates an embodiment of a non-contacting caliper sensor system 2 that includes upper and lower sensing scanner enclosures or heads 4 and 6, which are positioned on opposite sides of web or sheet 8 that is traveling in the machine direction (MD). The lower surface 11 of upper enclosure 4 and the upper surface 15 of lower enclosure 6 define a measurement gap or channel 54 through which web 8 travels. If the caliper measurement is to be performed in a scanning manner across the web 8 in the cross direction, the heads are aligned to travel directly across from each other as they traverse the moving web. In a preferred embodiment, upper head 4 includes a first optical displacement sensor 10 that gauges the perpendicular distance between lower surface 11 to the top surface of moving web 8. Similarly, the lower head 6 includes a second optical displacement sensor 14 that gauges the perpendicular distance between the upper surface 15 to the bottom surface of moving web 8. The enclosure surfaces 11 and 15 that are adjacent to the first and second optical displacement sensors 10 and 14, respectively, define apertures 11 and 12. Purge air is used to prevent dust from entering the scanner heads through these apertures. A suitable optical displacement sensor is the confocal imaging displacement sensor, model CL-3000 from Keyence Corporation.

[0024] In addition to optical displacement sensors, laser-based triangulation devices, nuclear, IR, RF, radar or microwave radiation-based device, acoustic-based systems, pneumatic-based devices, can be employed.

[0025] Caliper sensor system 2 further comprises a magnetic displacement or distance measurement mechanism for determining the distance between the upper and lower heads. The mechanism includes an electromagnetic coil 18 that is positioned in upper head 4 and first and second magnetic sensors 20, 22 that are positioned in lower head 6. The two magnetic sensors are positioned in tandem and are aligned with the coil along an axis. The electromagnetic coil is connected to a source of direct or alternating current to generate a magnetic field that is measured by the pair of magnetic sensors. Instead of using an electromagnetic coil and associated driving current source, a permanent magnet can be used as the source of magnetic field in upper enclosure 4.

[0026] In the configuration shown in Fig. 1, coil 18 is driven by an alternating current source 24 which also generates a reference signal 42 to demodulation circuits 30 and 32. When the magnetic field is generated by coil 18, fluxgate magnetic sensors 20 and 22 generates signals that are sent to demodulation circuits 30 and 32, respectively. A suitable fluxgate magnetic sensor is model DRY 425 from Texas Instruments. Typically, fluxgate sensors will only measure magnetic fields of up to 2 mT; it is preferable to maximize the magnetic field at the first fluxgate 20 so that it is close to this limit in order to reduce the possibility of an external field interfering with the desired signal. The demodulated outputs are digitized in analog-to-digital converters (ADCs) 36 and 38 and sent to computer 40. Alternatively, the signals from the magnetic sensors can undergo self-demodulation, without relying on the sync signal 42, to yield the demodulated outputs. The two measured voltages from the magnetic sensors 20 and 22 are proportional to the magnetic fields. The computer, which includes a microprocessor and memory that contains a lookup tables and / or parametric equations, analyzes that two measured voltages and applies the two optical displacement values to calculate the caliper of web 8.

[0027] One method of analyzing the data and obtaining displacements is to utilize ratios or differences in the two voltages. Magnetic flux density at a point along the axis of the coil can be calculated with the Biot-Savant relationship and is proportional to the inverse cube of the distance from the coil along the coil axis. Therefore, the ratio of the magnetic flux density at the positions of the two sensors is related to the cube of the ratio of the distances of the sensors from the coil. B = µ o NIAR 2< / ( 2( R 2< + Z 2< ) 3 / 2< ), where µ o = vacuum permeability, N = number of windings of the coil, I = electric current through the coil, A = area enclosed by the coil, Z = distance from the coil to the sensor, R = radius of coil loops. Thus B 1 / B 2 = {(R 2< + Z 2 2< ) / (R 2< + Z 1 2< )} 3 / 2< . The subscripts 1 and 2 refers to the measured magnetic flux density and coil-sensor distance for sensors 20 and 22. Since the distance between the sensors is fixed, where Z 2 is related to Z 1 by the sensor separation distance, the coil-sensor distance can be calculated through the above relationship and changes in the coil-sensor distance obtained from changes in the magnetic flux density ratio. Changes in the coil-sensor distance can then be used to correct for the changes in the separation distance of the upper and lower heads of the optical displacement devices of the non-contacting caliper sensor. Similarly, a relationship utilizing the difference between the magnetic flux densities at the two sensor locations can be used to calculate the coil-sensor distance and changes in the coil-sensor distance.

[0028] Fig. 2 illustrates the operation of caliper sensor system 2 in measuring the thickness of a web 8 that consists of a metal substrate 26 which is coated with an anode or cathode layer 28. Optical displacement sensor 10 measures the distance l 1 from aperture 12 to the surface of coating 28 and optical displacement sensor 14 measures the distance l 2 from aperture 16 to metal substrate 26. For illustrative purposes, the lower surface of sensor 10 is positioned at aperture 12 and therefore is co-planar with lower surface 11 of the top enclosure 4 and similarly the upper surface of sensor 14 is positioned at aperture 16 and therefore is co-planar with upper surface 15 of the lower enclosure 6.

[0029] In this configuration, the electromagnetic coil 48 has a helical structure and the two magnetic sensors 20, 22 are positional coaxially with the coil. It should be noted that in the case of a fluxgate magnetic sensor, the coil therein is very small relative to the electromagnetic coil 48. In designing the electromagnetic coil or permanent magnet, it is often preferred to choose a small one such that the field decays rapidly with distance in order to obtain the highest sensitivity to gap changes. The schematic depictions of magnetic sensors 20 and 22 are enlarged. The coil 48 is typically separated from magnetic sensor 20 by 5 to 25 mm and from the magnetic sensor 22 by 10 to 25 mm. Suitable coils are made of thin copper wire of approximately AWG 30 which is wound in a plastic bobbin and inserted into ferrite such that the back of the coil has ferrite and the front has no ferrite. The thickness of web 8 is equal to Z minus 1 2 and 1 2 .

[0030] The magnetic sensors 20, 22 are concentric with electromagnetic coil 48 so that the measurement axis of the dual magnetic sensors is collinear with the axis of symmetry of the coil. The coil can be circular in shape; it has been demonstrated that oval shaped coils may result in magnetic measurements with improved spatial resolution in one dimension.

[0031] The web 8 consists of a coated metal substrate such as electrode-coated metal foils used in the fabrication of anodes and cathodes for lithium ion electrochemical cells and batteries. The web 8 includes an aluminum or copper foil 26 that is coated with an electrode coating 28. The foil is typically 9 to 50 µm thick and the electrode coating ranges from 75 to 400 µm in thickness on one or both sides of the foil so that a double-side coated electrode can have a caliper of up to 850 µm with most being typically about 250 microns in thickness. For anodes the electrode coating includes graphite and for cathodes the electrode coating includes a lithium metal oxide such as LiCoO 2 . Electrodes are typically coated on both sides of a foil and the electrode coatings also include binders and conductivity enhancers.

[0032] The 1 KHz modulation / demodulation scheme produces a signal with high signal to noise while filtering the effect of interfering magnetic fields either static or time varying. The ratio (or difference) of the two magnetic sensor outputs cancels the effect of variation in current flowing through the coil and provides a highly stable and reliable measurement. It should be noted that higher frequencies will cause the effects associated with the conductive sheet to be more pronounced and noticeable. In contrast, lower frequencies produce less interference but will result in slower responses from the demodulation circuit. In the case of a DC field, one fluxgate sensor is typically used to measure any interfering magnetic field and can be used to cancel the effect. DC fields are more susceptible to interference from surrounding machinery.

[0033] The caliper of a moving sheet 22 that travels between two heads 62, 64 is determined by making the optical displacement measurement, d (optical), and inductive measurement, d (inductive). Thereafter, the thickness (t) of sheet 22 is calculated as being the difference between the two measurements with a constant offset, that is: t=d (inductive)-d (optical)-C. The offset constant is determined by calibration that is preferably conducted by taking a zero measurement when the sensor is offsheet, that is, when there is no sheet between the heads. The constant is determined by measuring something of known thickness as previously described. If the head separation varies slowly due to mechanical forces or thermal changes, an operator can periodically calculate the offset by scanning over a tile of known thickness during a standardization process. In addition, the standardization procedure can be used to detect abnormal conditions, such as if the optical sensors get dirty.

[0034] Instead of or in addition to employing optical displacement sensors 10, 14 to measure caliper, the scanner heads can serve as platforms for carrying sensors to detect sheet properties such as moisture and basis weight in the case of paper or characteristics of plastics. These devices typically use infrared, near-infrared and microwave radiation. Suitable sensors are described in U.S. Pat. No. 9,182,360 to Tixier and Hughes, U.S. Pat. No. 8,527,212 to Hughes and Tixier and U.S. Pat. No. 7,298,492 to Tixier, which are incorporated herein by reference.

[0035] Fig. 3 illustrates a scanning sensor system 60 wherein upper and lower scanner heads 64 and 66 are mounted on the elongated upper 63 and lower 65 arms or members, respectively, of a C-frame 62. The rigid members are parallel to each other. The frame 62 is equipped with a translation mechanism 69 which is configured as a linear slide to which the C-frame is movably secured. The upper head 64 incorporates the first optical displacement sensor 10 and coil 18 and lower head 66 incorporates the second optical displacement sensor 14 and dual magnetic sensors 20, 22 as shown in Fig. 1. The measurement channel between the heads accommodates the sheet of material being. The heads move back and forth along the cross direction (CD) as the sheet is monitored.

[0036] Instead of employing upper and lower heads that are secured to the distal ends of the elongated members 63 and 65, the sensor components can be integrated into or attached directedly to the members of the C-frame. For instance, a permanent magnet can be secured to upper member 63 and corresponding fluxgate sensors mounted to the lower member 65. Similarly, confocal displacement sensors can be mounted directly to the members.

[0037] A feature of affixing the heads on the arms of the C-frame structure is that the dual magnetic sensors can be calibrated without removing them from the lower head. By applying different levels of force on the upper head 64, the distance between the two heads will vary. In particular, weights are placed on the upper head 64 incrementally to cause the distance between the head to decrease. A target sample of known thickness is positioned between the heads and the optical displacement sensor devices on the heads can be utilized to measure the distance between the heads simultaneously with the coil-sensor measurements to calibrate the coil-sensor distance changes.

[0038] Fig. 4 illustrates a scanning sensor system 70 that includes dual head scanner heads 80, 82 measures the thickness or other properties during continuous web production. This scanning system is particularly suited for monitoring of wide webs or sheets such as during paper production where the paper can be more than ten meters wide. The upper head 80 and lower head 82 are supported by two transverse beams 72 and 74, respectively. The operative faces of the heads define a measurement gap 78 that accommodates sheet 76 which moves in the MD. Upper head 80 incorporates the first optical displacement sensor 10 and coil 18 and lower head 82 incorporates the second optical displacement sensor 14 and dual magnetic sensors 20, 22 as shown in Fig. 1. The cross directional movement of the dual scanner heads is synchronized with respect to speed and direction so that they are aligned with each other.

[0039] For the scanner sensor system 70, the magnetic displacement sensor is calibrated off-line before the components are incorporated into the upper and lower heads. For example, the dual fluxgate magnetic sensors can be secured to a stationary platform while the electromagnetic coil is mounted on a translation stage. The magnetic sensors and coil remain aligned as the translation stage is moved. An optical encoder or an interferometer measure the distance between the coil and the stationary platform.

[0040] Fig. 5 is a graph of magnetic flux density vs. distance along the centerline calculated from the Biot-Savart law for a coil with 178 turns and having a diameter of 1 cm. This data, along with the saturation level of the chosen magnetic detector, defines the required design parameters for the coil and coil-sensor distance.

[0041] The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.

Claims

1. A magnetic absolute displacement sensor for a system (2) for determining the thickness of a sheet of material wherein the sheet has a first side and a second side which comprises: a first member (4) disposed adjacent to the first side of the sheet of material, wherein the first member having means for producing a magnetic field (18, 24); a second member (6) disposed adjacent to the second side of the sheet of material, wherein the second member having a first magnetic senor (20) that detects the magnetic field and generates a first electrical signal and a second magnetic sensor (22) that detects the magnetic field and generates a second electrical signal; means for analyzing the first electrical signal and second electrical signal (40) to determine changes in a distance between the first and second member (4,6); wherein the first magnetic sensor (20) and second magnetic sensor (22) are positioned in tandem, wherein the means for producing a magnetic field (18) comprises an electromagnetic coil or permanent magnet and wherein the first and second magnetic sensors (20, 22) and the electromagnetic coil or permanent magnet are oriented along an axis.

2. A system (2) for determining the thickness of a sheet of material wherein the sheet has a first side and a second side, the system (2) comprising the magnetic absolute displacement sensor of claim 1 and further comprising a third sensor (10, 14) that measures a property or characteristic of the sheet of material.

3. The magnetic absolute displacement sensor of claim 1 wherein the first member (4) has a first mounting head and the second member (6) has a second mounting head and the two mounting heads define a gap (54) in which the sheet of material is positioned and the means for analyzing the first electrical signal and the second electrical signal (40) determines the distance of the gap (54).

4. A system (2) for determining the thickness of a sheet of material wherein the sheet has a first side and a second side, the system (2) comprising the magnetic absolute displacement sensor of claim 3 and further comprising a first distance sensor (10) in the first mounting head operative to determine a first distance between the first distance sensor (10) and the first side of the sheet of material and a second distance sensor (14) in the second mounting head operative to determine a second distance between the second distance sensor (14) and the second side of the sheet of material.

5. The magnetic absolute displacement sensor of claim 1 wherein the first magnetic sensor (20) comprises a first fluxgate sensor and the second magnetic sensor (22) comprises a second fluxgate sensor.

6. The magnetic absolute displacement sensor of claim 1 wherein the means for analyzing the first electrical signal and second electrical signal (40) analyzes the ratio of or the difference between the first and second electrical signal to derived the distance between the first and second members (4, 6).

7. The magnetic absolute displacement sensor of claim 1 having a slidably movable frame (62) having (i) an upper elongated rigid member (63) and (ii) a lower rigid elongated member (65) that is parallel to the upper elongated rigid member (63).

8. The magnetic absolute displacement sensor of claim 1 wherein the sheet of material comprises a metal layer.

9. A method of using a magnetic absolute displacement sensor for determining the thickness of a sheet of material having a first side and a second side that comprises: generating a magnetic field (18, 24) in a first member (4) disposed adjacent to the first side of the sheet of material (8); providing a first magnetic sensor (20) and a second magnetic sensor (22) in a second member (6) disposed adjacent to the second side of the sheet of material (8), wherein the magnetic sensors (20,22) detect the magnetic field (18, 24); wherein the first magnetic sensor (20) and second magnetic sensor (22) are positioned in tandem, wherein the means for producing a magnetic field (18) comprises an electromagnetic coil or permanent magnet and wherein the first and second magnetic sensors (20, 22) and the electromagnetic coil or permanent magnet are oriented along an axis generating a first electrical signal by the first magnetic sensor (20) and a second electrical signal by the second magnetic sensor (22); analyzing the first electrical signal and second electrical signal (40) to determine changes in a distance between the first and second member (4,6).

10. The method of claim 9 wherein the sheet of material (8) comprises a metal substrate (26) that is coated with an anode or cathode material (28).

11. The method of claim 9 wherein the electromagnetic coil (18) is used to generate a time varying magnetic field and wherein the first magnetic sensor (20), second magnetic sensor (22) and the electromagnetic coil (18) are coaxially aligned.

12. The method of claim 9 further comprising: providing a first distance sensor (10) on the first side of the sheet of material; determining the position of the first distance sensor (10) relative to the first side of the sheet of material with the first distance sensor (10); providing a second distance sensor (14) on the second side of the web (8); determining the position of the second distance sensor (14) relative to the second side of the web (8) with the second distance sensor (14); generating the magnetic field at a fixed location relative to the first distance sensor (10) on the first side of the web (8); positioning the first and second magnetic sensors (20, 22) at respective fixed first and second locations to the second distance sensor (14) on the second side of the web (8); measuring the magnetic field with the first and second magnetic sensors (20, 22); and determining the thickness of the web (8) from measurements of the magnetic field by the first and second magnetic sensors (20, 22).

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